1. Field of the Invention
The present invention relates to a magneto-optical disk having a multilayered magnetic film formed by stacking a plurality of magnetic layers and capable of performing a light-intensity-modulation direct overwriting operation.
2. Description of the Related Art
In recent years, a requirement for raising the transfer rate has arisen in the field of the magneto-optical recording and reproducing technical field. To meet the foregoing requirement, a variety of techniques have been put into practical use. Among the practical techniques, light-intensity-modulation direct overwrite has been known as a representative technique.
When a signal is recorded on a magneto-optical recording medium by the light intensity modulation method, a weak DC magnetic field is generally applied to the magneto-optical recording medium to modulate a laser beam depending upon whether or not a signal exists so as to apply the modulated laser beam to the magneto-optical recording medium. Therefore, the magneto-optical recording and reproducing apparatus adapted to the light intensity modulation method must renew recording after a recorded portion has been erased when a signal is recorded on a portion on which a previous signal has been recorded. Therefore a signal cannot directly be written over a recorded portion, that is, so-called direct overwrite cannot be performed.
Therefore, a major portion of magneto-optical recording and reproducing apparatuses put into practical use must initially perform an erasing operation prior to performing an operation for recording a new signal. Thus, there arises a problem in that at least time required for the magneto-optical recording medium to rotate at least two times is required. It leads to a fact that the transfer rate cannot be raised.
The light-intensity-modulation direct overwrite has been contrived to overcome the above-mentioned problem, the light-intensity-modulation direct overwrite being a technique adapted to the light intensity modulation method and enabling direct overwrite to be performed.
A specific method of the light-intensity-modulation direct overwrite has been disclosed in Japanese Patent Laid-Open No. 62-175948. The foregoing method is arranged such that a magneto-optical recording medium is formed by stacking two magnetic layers having different magnetic characteristics in such a manner that the switched connection is established between the two layers. A recording and reproducing apparatus according to the foregoing method includes an optical system which is capable of applying a laser beam controlled to a binary value consisting of a high level and a low level when a recording operation is performed; a recording magnetic field generating unit of a type for use in a usual magneto-optical recording and reproducing apparatus; and an external magnetic field generating unit which is capable of inverting only one of two magnetic layers at room temperature, that is, a so-called initializing magnet.
However, the above-mentioned method attempted to realize light-intensity-modulation direct overwrite must be provided with the initializing magnet for generating an initializing magnetic field of several kOe. The foregoing structure raises a problem when the magneto-optical recording and reproducing apparatus is designed.
Accordingly, reduction in the initializing magnetic field has been attempted by a method disclosed in, for example, Japanese Patent Laid-Open No. 1-185853. As a result, the initializing magnetic field has been reduced to about 2 kOe, which is considerably larger than the recording magnetic field which is about 300 Oe. The large initializing magnetic field prevents practical use of the above-mentioned method.
On the other hand, another method has been investigated individually from the foregoing attempt to reduce the initializing magnetic field in order to essentially overcome the above-mentioned problem. The method is attempted to enable light-intensity-modulation direct overwrite to be performed without use of the initializing magnetic field. As a result, magneto-optical recording mediums, disclosed in Japanese Patent Laid-Open No. 63-268103 and Japanese Patent Laid-Open No. 3-219449, have been contrived.
The magneto-optical recording mediums have the basic structure formed by adding a magnetic layer serving as the initializing magnet attributable to switched connection to the magneto-optical recording medium disclosed in Japanese Patent Laid-Open No. 62-175948. When a magneto-optical recording medium of the foregoing type is used, the light-intensity-modulation direct overwrite can be performed without use of the initializing magnet.
However, the magneto-optical recording medium enabled to perform the light-intensity-modulation direct overwrite by adding the magnetic layer serving as the initializing magnet attributable to the switched connection cannot simultaneously satisfy the required characteristics, including the sensitivity to the recording magnetic field, the quality of the signal and stability of the overwriting operation. The known magneto-optical recording mediums capable of performing light-intensity-modulation direct overwrite cannot simultaneously satisfy the above-mentioned characteristics and practical use has not been realized.